Daylight saving time (DST) begins at 2 a.m., local time, on Sunday, March 10. Most U.S. residents dutifully set their clocks ahead one hour before they went to bed Saturday night. People in most of Arizona, Hawaii and in the U.S. territories of American Samoa, Puerto Rico and the Virgin Islands, however, will leave their clocks alone. These locations never sway from standard time within their time zones.

Why do we have daylight saving time? Daylight saving time decreases the duration of daylight we experience in the early morning hours and increases the amount of daylight available in the evening, when most of us are awake.

The ability for countries to implement daylight saving time was made possible by the establishment of nationally standardized times less than four decades earlier. Prior to standardized time, time was kept locally, and clocks varied from town to town; this created problems for railroad companies, which needed uniform times to coordinate train schedules.

In 1880, the British government ordered that the entire country adopt Greenwich Mean Time. American and Canadian railroad companies soon created Standard Railway Time, which created five standardized North American time zones. Most U.S. localities agreed to adopt Standard Railway Time in 1883, though it was not made mandatory until March 1918 with the passage of the Standard Time Act of 1918.

The Standard Time Act also implemented daylight saving time in the United States, but this section of the law was repealed the following year due to the end of the war. The decision to implement daylight saving time was placed in the hands of local jurisdictions, where it remained—aside from a three and a half-year period during World War II—until 1966.

Congress established uniform start and end dates (the last Sunday in April and last Sunday in October) with the passage of the Uniform Time Act of 1966. The act gave states the right to opt out of daylight saving time, but took away the right from local jurisdictions.

The start and end dates of daylight saving time has varied over the years. In 1973, in an effort to conserve fuel during the Arab oil embargo, Congress extended daylight saving time an extra two months. The start date was moved to the first Sunday in April in 1986. Starting in 2007, the start date was moved back to the second Sunday in March, and the end date was moved to the first Sunday in November, extending the period of daylight saving time to just under eight months.

Daylight saving time is based on the assumption that it saves energy. The 2007 changes sparked a national debate on the topic. Angela Webber of The Oregonian cites two studies—one performed by the California Energy Commission and one by the National Bureau of Economic Research (NBER)—that found that daylight saving time does not save energy.

In fact, according to the NBER study, "Our main finding is that—contrary to the policy’s intent—DST increases residential electricity demand.” Both studies found that the energy people save from not using as much power at night is offset by the increased energy used during the darker mornings of autumn.

The extension of daylight saving time, he argues, was instituted to benefit retailers: “When we have an hour of sunlight after work, Americans tend to go shopping.” In addition, candy producers pushed for DST to end after Halloween so trick-or-treaters would have an extra hour of daylight.

In 1986, Congress found other advantages, such as “more daylight outdoor playtime” for children, as well as “greater utilization of parks and recreation areas, expanded economic opportunity through extension of daylight hours to peak shopping hours and through extension of domestic office hours to periods of greater overlap with the European Economic Community.”

But not everyone agrees that more daylight in the evening is a bonus. As Mosley points out, farmers and other early birds who get up before the sun “may have to operate in the dark a while longer before daybreak.”

That was the case when Emergency Daylight Saving Time Energy Conservation Act of 1973 took effect on Jan. 6, 1974. Millions of people found themselves “getting up in what did indeed seem like the middle of the night,” Time magazine reported at the time. A surge of traffic accidents involving children going to school in the dark was reported, along with a number of other calamities.